What % of O2 is a patient getting with a nasal cannula?

I'm doing a case, i have patient on 3L NC. in the fio2 section the MDA sees that i've been writing .27

He asks me 'how much oxygen is this patient getting'. I replied that at 3L he is getting 27%.

I told him I had read a formula on the net for NC oxygen concentration and that is:

# L of O2 * 6 + 21 (room air) --- thus: 3*2+21 = 27

he proceeds to tell me that Nasal cannulas provide 40-60% O2 i believe. then a face mask i think he said 60-80* and ofcourse a nonrebreather is 100%.

my question is i guess also besides a nasal cannula, how bout a face mask??

he laughed at my formula and proceeded to make me feel like an asshole in front of CRNA.


i've tried google searching for articles but was unable to find anything. I would love some scholarly advice.